Electric power steering devices of the aforesaid type employ a worm shaft as a driving gear and a worm wheel as a driven gear (see, for example, Japanese Unexamined Patent Publication No. 2001-108025).
In general, the worm shaft and the worm wheel should be meshed with each other with proper backlash for reduction of a torque loss. However, when vibrations are inputted from a road surface to tires while traveling under bad road conditions, for example, rattle occurs between the worm shaft and the worm wheel due to the backlash.
In order to minimize the torque loss and generation of the rattle, the amount of the backlash should be adjusted within a predetermined machining accuracy. That is, a proper combination of the worm shaft and the worm wheel is selected according to variations in the dimensional accuracies of the worm shaft and the worm wheel to provide a proper combinational accuracy when the worm shaft and the worm wheel are assembled (a so-called matching operation). However, this operation is cumbersome, thereby increasing production costs.
The aforementioned worm wheel is usually formed of a synthetic resin. In this case, the backlash amount is liable to be changed by the wear of the teeth of the worm wheel and the expansion and contraction of the worm wheel due to a temperature change, moisture absorption and the like.
To cope with this, it is proposed that the worm shaft is supported so as to be offset toward the worm wheel, and resiliently biased toward the worm wheel to suppress the torque loss and eliminate the backlash (see, for example, Japanese Unexamined Patent Publication No. 2001-146169). More specifically, a corrugated plate spring having an arcuate shape is provided between a bearing which supports a support shaft of the worm shaft and a bearing retention hole of a housing, and the worm shaft is biased toward the worm wheel by the biasing force of the corrugated plate spring.
However, where the corrugated plate spring is used for generating the biasing force radially of the bearing, the corrugated plate spring should be formed with a sufficient amount of undulations. As a result, the device has an increased in size. On the other hand, it is demanded to increase durability of the device.
Similar problems are also found in electric power steering devices utilizing a common gear mechanism other than the worm shaft and the worm wheel.
It is an object of the present invention to provide an electric power steering device which permits to bias a driving gear resiliently toward a driven gear thereof accompanied by reduction in size and improvement in durability.